This invention relates to a dry etching method applicable to the fabrication of semiconductor devices. More particularly, it relates to a method of selective etching of GaAs/AlGaAs in a process step during formation of a gate recess of a high electron mobility transistor (HEMT) without employing a chlorofluorocarbon (CFC) gas.
A monolithic microwave IC in which a GaAs MES-FET (metal semiconductor field effect transistor) is integrated on a single substrate, has characteristics such is high speed high frequency response, low noise or low power consumption, and is recently used as a device for mobile communication or satellite communication.
In 1980, the high electron mobility transistor (HEMT) has been developed from researches aimed at a higher operating speed of the above-mentioned GaAs MES-FET. The HEMT is a device in which advantage is taken of a two-dimensional electron gas at a hetero junction of the GaAs compound semiconductor being moved at a high velocity without being subject to scattering by impurities. Researches towards realization of a high integration in HEMT are proceeding currently and demands are raised for higher precision and higher selectivity ratio of the dry etching technique for realization of the higher integration.
The process of selective etching of a GaAs/AlGaAs stacked system for forming a gate recess is an important technique of determining a threshold voltage of a hereto-junction FET such as HEMT or hereto MIS structure FET. It is because the concentration of impurities and thicknesses etc. in the lower AlGaAs layer are preset so that an FET having a reasonable threshold voltage may be produced by removing only the upper GaAs layer. The method for selective etching of the GaAs layer on the AlGaAs layer may be typified by a method employing a gas mixture composed of chlorofluorocarbon gases (CFC gases), such as CCl.sub.2 F.sub.2, and rare gases. The reason is that the GaAs layer is removed mainly in forms of chlorides of Ga and As, and fluoride of As, and that ALF.sub.3 having a low vapor pressure is formed on the surface on exposure of the underlying AlGaAs layer to lower the etching rate to achieve a high selectivity ratio.
For example, a report has been made of an example of achieving a selectivity ratio of 200 using a CCl.sub.2 F.sub.2 /He mixed gas in e.g. Japanese Journal of Applied Physics, Vol. 20, No. 11 (1981) p. L847 to 850.
However, the above-mentioned selective dry etching method suffers from the following disadvantages.
First, the above-mentioned CFC gases, such as CCl.sub.2 F.sub.2, are thought to cause destruction of an ozone layer surrounding the earth in a known manner, so that a ban will be placed on the production and use of these gases in the near future. It is therefore incumbent to develop substitute gases and the technology of application of these substitute gases in the field of dry etching.
On the other hand, the CFC gasses tend to produce a large quantity of the fluorocarbon-based polymer in an etching reaction system. This polymer, while being deposited on the pattern sidewall to display sidewall protection effects to contribute to anisotropic etching, tends to produce instability of the etchrate and a deteriorated level of pollution by particles.
Besides, annealing at a temperature on the order of 300 .degree. C. becomes necessary for remedying damages by ion radiation during etching.